Exhaust state control device for fuel cell for mobile unit

ABSTRACT

An exhaust state control device for a fuel cell for a mobile unit includes an exhaust gas temperature sensor ( 17 ) that measures the temperature of exhaust gas in a discharge passage that discharges the exhaust gas from the main body of the fuel cell, and an ambient temperature sensor ( 19 A) that measures the temperature of ambient air to which exhaust gas is to be discharged. If, based on the difference between the exhaust gas temperature and the ambient temperature, it is determined that white smoke is generated, it is determined whether a condition to reduce white smoke is satisfied. When a first condition is satisfied, a process to reduce white smoke is activated. When a second condition, which requires further reduction of white smoke than under the first condition, is satisfied, a process to suppress the generation of white smoke is activated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique to control the exhauststate for a fuel cell for a mobile unit.

2. Description of the Related Art

In fuel cells, a reaction generates electricity and discharges acorresponding amount of generated water. Especially in fuel cells forautomobiles, a larger amount of water is generated as the vehicletravels a longer distance. Most fuel cells for automobiles are of asolid polymer type, which basically operates at low temperatures. Acommon problem with the fuel cells is the treatment of the generatedwater, that is, preventing the generated water from freezing on the roadin cold regions or from splashing toward any following vehicles.

Depending on the ambient air conditions and the driving conditions,however, it is also necessary to suppress the generation of white smokefrom the exhaust port of a discharge passage for off gas. Generation ofthe white smoke is not desirable from the standpoint of themerchantability of the automobiles. In some instances, the impact of theautomobiles on the surroundings due to the white smoke should be takeninto account. In view of the above, proposals have been made to suppressthe white smoke in fuel cells for vehicles in Japanese PatentApplication Publication No. 7-169498 (JP-A-7-169498) and Japanese PatentApplication Publication No. 2001-185199 (JP-A-2001-185199).

To reduce the white smoke, in general, the fuel cells for vehicles haveadopted means for cooling or heating the off gas beyond the temperaturerange where white smoke is easily generated, means for suppressing theamount of scavenged air, and so forth. Therefore, energy is required toreduce the white smoke, which contradicts the requirement to improve thepower generation efficiency.

That is, the fuel cells described in the above documents estimate thegeneration of white smoke mainly based on the difference between theexhaust air temperature and the ambient temperature. Then, a white smokereduction process is executed according to the estimation results. Ifthe white smoke reduction process is executed frequently, the fuelefficiency is reduced due to the heating, the energy efficiency isreduced, the output is reduced due to the suppressed amount of scavengedair, etc., to a larger degree.

As a result of a more detailed examination of the circumstances wherethe white smoke is generated, the following may be pointed out. Thewhite smoke generated is not noticeable while the vehicle is in motionbecause of the diffusion effect of the head wind, but is noticeablewhile the vehicle is stationary or traveling at low speeds. Thevisibility of the white smoke varies in accordance with theenvironmental conditions around the vehicle, such as whether it isdaytime or nighttime and whether it is sunny or rainy. The white smokegenerated affects the vision of the operator when backing the vehicle.

SUMMARY OF THE INVENTION

The present invention provides an exhaust state control device for afuel cell for a mobile unit, such as a vehicle, powered by a fuel cell,that determines the necessity for white smoke reduction based on thetraveling conditions of the mobile unit, the operating state of themobile unit, or the environmental conditions to efficiently suppress thewhite smoke.

An aspect of the present invention is directed to an exhaust statecontrol device for a fuel cell for a mobile unit that detects a state ofexhaust gas from a fuel cell provided in the mobile unit. The exhauststate control device for the fuel cell includes: a white smokegeneration determination section that determines whether white smoke isgenerated based on relationship between a state of the exhaust gas and astate of the ambient air; a motion sensor that detects movement of themobile unit; a condition determination section that determines whether acondition to reduce white smoke is satisfied based on a detection signalfrom the motion sensor when the white smoke generation determinationsection determines that white smoke is generated; and a control sectionthat activates a process to reduce white smoke if the conditiondetermination section determines that a predetermined condition issatisfied.

The exhaust state control device for a fuel cell for a mobile unit mayfurther include: an operation state sensor that detects an operatingstate of an operating section that operates the mobile unit; and asensor that senses a surrounding environmental state of the mobile unit,and the condition determination section may determine whether thecondition to reduce white smoke is satisfied based on at least one ofthe detected moving state of the mobile unit, the detected operatingstate of the operating section that operates the mobile unit, and thedetected surrounding environmental state of the mobile unit.

In the exhaust state control device for a fuel cell for a mobile unit,the control section may activate a process to reduce white smoke whenthe condition determination section determines that a firstpredetermined condition is satisfied, and activate a process to furtherreduce generation of white smoke than when the first predeterminedcondition is determined to be satisfied, if the condition determinationsection determines that a second predetermined condition is satisfied.

The exhaust state control device for a fuel cell for a mobile unitaccording to the above aspect may further include: an exhaust gastemperature sensor that measures a temperature of the exhaust gas in adischarge passage that discharges the exhaust gas from a main body ofthe fuel cell; and an ambient temperature sensor that measures atemperature of the ambient air to which the exhaust gas is to bedischarged, and the white smoke generation determination section maydetermine whether white smoke is generated according to the differencebetween the exhaust gas temperature and the ambient temperature.

The exhaust state control device for a fuel cell for a mobile unitaccording to the above aspect may further include: an exhaust gastemperature sensor that measures a temperature of the exhaust gas in adischarge passage that discharges the exhaust gas from a main body ofthe fuel cell; an ambient temperature sensor that measures a temperatureof the ambient air to which the exhaust gas is to be discharged; and anambient humidity sensor that measures a humidity of the ambient air towhich the exhaust gas is to be discharged, and the white smokegeneration determination section may determine whether white smoke willbe generated according to the difference between the exhaust gastemperature and the ambient temperature and the ambient humidity.

According to the above aspect, if it is determined that white smoke willbe generated, white smoke reduction can be performed in different levelsby determining whether the first condition is satisfied or the secondcondition, which requires greater reduction of white smoke than thefirst condition does, is satisfied. In general, the process to furtherreduce white smoke lowers the efficiency of the fuel cell for a mobileunit more than the process to reduce white smoke does. Thus, the exhauststate control device for a fuel cell for a mobile unit can improve theefficiency of the fuel cell for a mobile unit by determining in detailthe conditions under which white smoke is generated to perform the whitesmoke reduction process in different levels.

In the above aspect, the first condition may be satisfied if either oneof the conditions, where a moving speed of the mobile unit is determinedto be at or below a predetermined value and where a visibility of whitesmoke around the mobile unit is determined to be higher than that in areference environment, is satisfied; and the second condition may besatisfied if both the conditions, where the moving speed of the mobileunit is determined to be at or below the predetermined value and wherethe visibility of white smoke around the mobile unit is determined to behigher than that in the reference environment, are satisfied. Accordingto the above aspect, the exhaust state control device for a fuel cellfor a mobile unit can determine in detail the circumstances where whitesmoke is generated by defining the first condition and the secondcondition which requires greater reduction of white smoke than the firstcondition does.

In the above aspect, the condition, where the moving speed of the mobileunit is determined to be at or below the predetermined value may includea state where the mobile unit is stationary. Greater reduction of whitesmoke is occasionally required when the vehicle is stationary than whenit is moving. In the above aspect, the moving speed of the mobile unitmay be an absolute speed of the mobile unit or a relative speed betweenthe mobile unit and the ambient air around the mobile unit. Thegeneration and the visibility of white smoke are affected by therelative speed between the mobile unit and the ambient air.

In the above aspect, the mobile unit may be a vehicle; the firstcondition may be satisfied if either one of conditions, where a shiftlever is determined to be in a parking position and where a visibilityof white smoke around the mobile unit is determined to be higher thanthat in a reference environment, is satisfied; and the second conditionthe second condition may be satisfied if both the conditions, where theshift lever is determined to be in the parking position and where thevisibility of white smoke around the mobile unit can be determined to behigher than that in the reference environment, are satisfied.

In the above aspect; the first condition may be satisfied if either oneof conditions, where the white smoke generation determination sectiondetermines that white smoke is generated or a condition where a movingspeed of the mobile unit is at or below a predetermined value and wherea visibility of white smoke around the mobile unit is determined to belower than that in a reference environment, is satisfied; and the secondcondition may be satisfied if both the conditions, where the movingspeed of the mobile unit is determined to be at or below thepredetermined value and where the visibility of white smoke around themobile unit is determined to be higher than that in the referenceenvironment, are satisfied.

In the above aspect, if the conditions, where the white smoke generationdetermination section determines that white smoke is generated, andwhere the moving speed of the mobile unit is at or below thepredetermined value and the visibility of white smoke around the mobileunit is determined to be lower than that in the reference environment,are satisfied, the condition determination section may perform theprocess to reduce white smoke, in the case the condition determinationsection determined that the first predetermined condition has beensatisfied, in different level.

In the above aspect, the first condition may be satisfied if either oneof conditions, where a vision of an operator who operates the mobileunit is determined to be affected by white smoke and where a visibilityof white smoke around the mobile unit is higher than that in a referenceenvironment, is satisfied; and the second condition maybe satisfied ifboth the condition, where the vision of the operator who operates themobile unit is determined to be affected by white smoke and where thevisibility of white smoke around the mobile unit is higher than that inthe reference environment, are satisfied. According to the above aspect,the exhaust state control device for a fuel cell for a mobile unit candetermine in detail the circumstances where white smoke is generated bydefining the first condition and the second condition which requiresgreater reduction of white smoke than the first condition does.

In the above aspect, the reference environment may include at least oneof a daytime environment and a non-rainy environment. The visibility ofwhite smoke is considered to be lowest in daytime or non-rainyenvironments.

In the above aspect, the mobile unit may be a vehicle; the operatingstate of the operating section may be determined based on whether aheadlight is on; the surrounding environmental state may be determinedbased on at least one of: whether a darkness detection signal for anautomatic light is on; whether a current time is after a sunset andbefore a sunrise; whether a signal from a global positioning systemcannot be received; whether a wiper is on; whether a rain sensing signalis on; and whether a detection value of the ambient humidity sensorexceeds a predetermined value; and the condition determination sectionmay determine that the reference environment is the daytime or non-rainyenvironment if at least one of the determined states above is satisfied.

In the above aspect, it may be determined that the white smokeinterferes with the vision of the operator when the mobile unit advancesin a discharging direction of the exhaust gas. The possibility that thewhite smoke will interfere with the vision of the operation increases ifthe white smoke is at the advance direction and the operator is lookingin the advance direction of the mobile unit.

In the above aspect, the mobile unit may be a vehicle; and it may bedetermined that the condition where the white smoke interferes withvision of the operator of the mobile unit is satisfied when at least oneof the shift lever is in a reverse position and the parking brake isoff.

In the above aspect, the process to reduce white smoke may be at leastone of a process to control the exhaust gas temperature and a process tocontrol a flow rate of the exhaust gas at an air electrode side of thefuel cell. The generation of white smoke can be controlled by theexhaust gas temperature and the flow rate of the exhaust gas at the airelectrode side.

In the above aspect, discharge of the exhaust gas at the air electrodeside may be controlled such that a limit of the flow rate of the exhaustgas decreases as the moving speed of the mobile unit decreases. Thegeneration of white smoke is reduced and the visibility of white smokeis lowered when the moving speed of the mobile unit is at or above apredetermined value.

In the above aspect, a pressure or the flow rate of the exhaust gas atthe air electrode side may be controlled such that the limit of the flowrate of the exhaust gas increases as the pressure of the exhaust gasincreases. To discharge the same amount in mass of exhaust gas, thevolumetric flow rate of the gas is decreased as the pressure of the gasis increased. Therefore, to discharge the same amount in mass of exhaustgas, the amount of water vapor to be discharged is reduced as thepressure of the exhaust gas is higher. To maintain approximately thesame amount of water vapor to be discharged, a larger amount in mass ofexhaust gas can be discharged when the pressure of the exhaust gas isincreased than when the pressure is not increased.

In the above aspect, the process to reduce the generation of whitesmoke, which is performed when the condition determination sectiondetermines that the second predetermined condition is satisfied,includes at least one of stopping scavenging air and shutting off ahydrogen supply pressure to the fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features and advantages of the invention willbecome apparent from the following description of example embodimentswith reference to the accompanying drawings, wherein like numerals areused to represent like elements and wherein:

FIG. 1 shows an example configuration of a fuel cell system;

FIG. 2 is a flowchart illustrating the operation of the fuel cellsystem;

FIG. 3 shows an example process of the white smoke reduction control;

FIG. 4 shows the classification of the white smoke reduction process;

FIG. 5 shows an example process of the white smoke reduction control;

FIG. 6 shows an example process of the white smoke reduction control;

FIG. 7 is a flowchart illustrating the operation of the fuel cellsystem;

FIG. 8 shows an example of the map showing white smoke reductionregions;

FIG. 9 is a configuration diagram of the fuel cell system;

FIG. 10 shows the concept of a vehicle equipped with the fuel cellsystem; and

FIG. 11 shows an example process of the white smoke reduction control.

DETAILED DESCRIPTION OF EMBODIMENTS

A fuel cell system according to a first embodiment of the presentinvention is described below with reference to FIGS. 1 to 3.

FIG. 1 shows an example configuration of a fuel cell system inaccordance with an embodiment of the present invention. As shown in FIG.1, the fuel cell system includes a fuel cell stack 1. The fuel cellsystem also includes, a hydrogen tank 7, which serves as its hydrogensystem that supplies hydrogen to the fuel cell stack 1; a pressureregulator 10, an anode gas passage 2, an anode off gas passage 3, ahydrogen pump 8, an anode off gas circulation passage 4, a gas-liquidseparator 12, which separates water from anode off gas; a drain tank 13,and an exhaust/drain valve 14. The fuel cell system additionallyincludes, as its air system that supplies air to the fuel cell stack 1,an air filter 11, a pump 9, a cathode gas passage 5, and a cathode offgas passage 6. The fuel cell system further includes, as its controlsystem and measurement system, an electronic control unit (ECU,equivalent to the exhaust state control device for a fuel cell for amobile unit) 15, a discharge gas temperature sensor 17, an ambienttemperature sensor 19A, an ambient humidity sensor 19B, and a stacktemperature sensor 20. As shown schematically in FIG. 1, the fuel cellsystem is mounted on a mobile unit such as a vehicle to function as apower source for the mobile unit.

The fuel cell stack 1 is composed of a plurality of cells stacked overeach other. Each cell is composed of an electrolyte membrane, an anode(fuel electrode), a cathode (air electrode), and a separator. Flow pathsfor hydrogen and air are formed between the anode and the cathode.

The hydrogen tank 7 supplies anode gas to the anode gas passage 2. Theanode gas supplied from the hydrogen tank 7 is adjusted to apredetermined pressure by the pressure regulator 10. Then, the anode gasis supplied from the anode gas passage 2 to the anode of the fuel cellstack 1.

Air drawn via the air filter 11 is supplied through the cathode gaspassage 5 to the cathode of the fuel cell stack 1 from outside the fuelcell system by the pump 9 (which may be an air compressor).

When the anode gas is supplied to the anode of the fuel cell stack 1,hydrogen ions are generated from hydrogen contained in the anode gas.Oxygen contained in the air is supplied to the cathode of the fuel cellstack 1. Then, in the fuel cell stack 1, an electrochemical reactionoccurs between the hydrogen and the oxygen that generates electricalenergy. In addition, at the cathode of the fuel cell stack 1, thehydrogen ions, generated from hydrogen, and oxygen are combined thatgenerates water.

Gas containing unreacted hydrogen and nitrogen and so forth permeatedfrom the cathode (hereafter referred to as “anode off gas”) isdischarged from the fuel cell stack 1 to the anode off gas passage 3.

The anode off gas discharged from the anode of the fuel cell stack 1passes through the anode off gas passage 3 and the anode off gascirculation passage 4, and again is supplied to the anode of the fuelcell stack 1 along with anode gas from the hydrogen tank 7. The anodeoff gas passage 3 supplies the anode off gas to the gas-liquid separator12. This allows the anode off gas to be supplied to the anode off gascirculation passage 4 after water is separated from the anode off gas.

Unreacted cathode gas (hereinafter referred to as “cathode off gas”) isdischarged from the fuel cell stack 1 to the cathode off gas passage 6.The cathode off gas contains the water generated by the fuel cell stack1 in the form of water vapor. The cathode off gas discharged from thecathode is discharged through the cathode off gas passage 6 to theambient air.

The ECU 15 is electrically connected to the pump 9, a driving motor (notshown) that drives the pressure regulator 10, the discharge gastemperature sensor 17, the ambient temperature sensor 19A, the ambienthumidity sensor 19B (equivalent to one of the sensors), and the stacktemperature sensor 20. The ECU 15 controls supply of the pump 9 and thedriving motor (not shown) for the pressure regulator 10. The ECU 15acquires the temperature of the cathode off gas detected by thedischarge gas temperature sensor 17. The ECU 15 acquires the ambienttemperature detected by the ambient temperature sensor 19A and theambient humidity detected by the ambient humidity sensor 19B. The ECU 15acquires the operating temperature of the fuel cell stack 1 measured bythe stack temperature sensor 20. The ECU 15 includes a CPU, a ROM, andso forth inside. The CPU executes various processes according to acontrol program stored in the ROM.

The discharge gas temperature sensor 17 detects the temperature of thecathode off gas that is discharged via the cathode off gas passage 6 tothe ambient air. The discharge gas temperature sensor 17 may be providedat any position in the cathode off gas passage 6. For example, thedischarge gas temperature sensor 17 may be provided in the vicinity ofan exhaust exit (tail end) of the cathode off gas passage 6 so as tomeasure the temperature of the cathode off gas immediately before beingdischarged to the ambient air.

The ambient temperature sensor 19A measures the temperature outside thefuel cell system, that is, the ambient temperature. Meanwhile, theambient humidity sensor 19B measures the ambient humidity. The stacktemperature sensor 20 measures the operating temperature of the fuelcell stack 1. The stack temperature sensor 20 may be directly attachedto the fuel cell stack 1, or may measure the temperature of the coolantof the fuel cell stack 1.

The vehicle is provided with a vehicle speed sensor 22 (equivalent toone of the sensors) that detects the moving speed of the vehicle, andvarious levers, operating buttons, knobs, pedals, and so forth thatallow a driver to operate the mobile unit. Examples of the variouslevers, operating buttons, knobs, pedals, and so forth include a shiftlever that designates the ratio between the rotational speed of a motordriven by a power source and that of a driving part (for example, awheel), a wiper lever that turns on and off a wiper, a knob of anin-vehicle device that receives a signal from a global positioningsystem (GPS) to provide an indication of the present location and time,guidance to a destination, and so forth, and a parking brake lever. Thevehicle in accordance with this embodiment has respective sensors thatsense operations of the various levers, operating buttons, knobs,pedals, and so forth by the driver (each equivalent to the sensor of thepresent invention), and executes control according to the operations.During this control, the ECU 15 monitors the operations of the varioussensors and so forth to recognize their operating states.

FIG. 2 is a flowchart of an exhaust state control process that isexecuted by the ECU 15. The process is implemented by a control programexecuted by the CPU. This process is executed periodically atpredetermined time intervals.

In the process, the ECU 15 first acquires the gas temperature at theexhaust exit, detected by the discharge gas temperature sensor 17 andthe temperature detected by the ambient temperature sensor 19A. Then,the ECU 15 determines whether the difference between the gas temperatureat the exhaust exit and the ambient temperature exceeds a predeterminedvalue T1 (S1). If the difference between the gas temperature at theexhaust exit and the ambient temperature does not exceed thepredetermined value T1 (NO in S1), the ECU 15 returns the process to S1.In this case, the ECU 15 may execute S1 and the subsequent steps after apredetermined amount of time has elapsed. Here, the predetermined valueT1 and the predetermined time may be set as factory default, or set bythe dealer or the user of the vehicle, for example. Incidentally, theECU 15 which executes the process in S1 may be equivalent to the whitesmoke generation determination section of the present invention.

On the other hand, if the difference between the gas temperature at theexhaust exit and the ambient temperature exceeds the predetermined valueT1 (YES in S1), the ECU 15 reads the traveling speed of the vehicle fromthe vehicle speed sensor 22 and reads the position of the shift lever(S2).

Then, the ECU 15 determines whether the vehicle speed is 0, ordetermines whether the shift lever is in the parking position (Pposition) (S3). At this time, the ECU 15 may determine whether thevehicle speed is 0 and whether the shift lever is in the P position. Theconditions in S3 may be equivalent to the first condition of the presentinvention.

Then, if the vehicle speed is not 0, or if the shift lever is not in theP position (NO in S3), the ECU 15 returns the process to S1. On theother hand, if the vehicle speed is 0, or if the shift lever is in the Pposition (YES in S3), the ECU 15 determines the surroundingenvironmental conditions (S4). The surrounding environmental conditionsinclude the following, for example. (1) A headlight is on (which may beequivalent to the operating state of the operating section of thepresent invention). (2) A darkness detection signal for an automaticlight is on (which may be equivalent to the environmental state of thepresent invention). Here, the “automatic light” refers to a headlightthat is turned on and off automatically. That is, the automatic lightgenerates a darkness detection signal to turn on the headlight when itscontroller senses that the brightness in its environment has fallenbelow a predetermined threshold. (3) It is after a sunset and before asunrise (which may be equivalent to the environmental state of thepresent invention). (4) A signal from the GPS cannot be received (whichmay be equivalent to the environmental state of the present invention).(5) The wiper is on (which may be equivalent to the operating state ofthe operating section of the present invention). (6) For a vehicleprovided with an automatic wiper, a rain sensing signal is on (which maybe equivalent to the environmental state of the present invention). (7)A detected ambient humidity h of the ambient humidity sensor 19B exceedsa predetermined value A (which may be equivalent to the environmentalstate of the present invention). Here, the predetermined value A may beset as factory default, or set by the dealer or the user of the vehicle,for example.

Then, the ECU 15 determines whether the surrounding environmentdetermined in S4 satisfies a predetermined condition (S5). Here, thepredetermined condition may be set as factory default, or set by thedealer or the user of the vehicle, for example. If none of theseconditions is satisfied (NO in S5), the ECU 15 executes the white smokereduction control (S6). In this case, the surrounding environmentalcondition is determined that it is daytime, that it is not rainy, andthat the ambient humidity is less than a predetermined value. Thus, itis determined that the visibility of the white smoke is low, andtherefore the process to reduce the generated white smoke is executed.

Here, the white smoke reduction control is a process that reduces theamount of scavenging air at the air electrode. For example, the ECU 15reduces the amount of scavenging air from the pump 9 by a predeterminedproportion. Alternatively, the cathode off gas may be heated by a heater(not shown) provided in the cathode off gas passage 6, for example.Conversely, the cathode off gas may be cooled by a heat exchanger (notshown) to a temperature close to the ambient temperature.

The cathode off gas may be heated or cooled based on conditions forwhite smoke that is generated, which are obtained from the relationshipbetween the ambient temperature and the gas temperature at the exhaustexit, for example. This is made possible by experimentally orempirically preparing a map of the relationship that “the differentialbetween the ambient temperature and the gas temperature at the exhaustexit is ΔT or less at a humidity of h,” and storing the map in a storagedevice of the ECU 15. Then, the ECU 15 can perform control so as to heator cool the cathode off gas such that the differential between theambient temperature and the gas temperature at the exhaust exit is ΔT orless at the ambient humidity.

On the other hand, if any of the above conditions is satisfied (YES inS5), the ECU 15 executes the white smoke suppression control (S7). Inthis case, the surrounding environmental condition is determined that itis nighttime, that the headlight is used, that it is rainy, or that theambient humidity exceeds the predetermined value. Thus, the visibilityof the white smoke is high, and therefore the process to reduce thegenerated white smoke is not enough and the white smoke suppressionprocess is executed. Incidentally, the white smoke suppression controlin the present invention may be equivalent to the process to furtherreduce generation of white smoke.

Here, the white smoke suppression control may include, for example,stopping the operation of the pump 9 (that is, stopping scavenging air),and suspending the operation of the fuel cell system by shutting off thehydrogen supply pressure with the pressure regulator 10.

As discussed above, the fuel cell system in accordance with thisembodiment can precisely determine whether white smoke reduction isrequired by determining the visibility of white smoke based on thetraveling conditions of the vehicle, the operating state of the vehicle,and the surrounding environmental conditions when the generation ofwhite smoke is highly possible in consideration of the relationshipbetween the temperature of cathode off gas to be discharged to theambient air and the ambient temperature. Then, the fuel cell system inaccordance with this embodiment executes a white smoke reduction processwhen the visibility of the white smoke is estimated to be notparticularly high, and executes a white smoke suppression process whenit is estimated to be high. That is, white smoke reduction is switchedaccording to the driving conditions of the vehicle and the surroundingenvironmental conditions. This contributes to precisely determiningwhether white smoke reduction is required, and to reducing wastefulenergy consumption and a decrease in the power generation efficiency.

In the first embodiment, as described in relation to S4 and S5 of FIG.2, the ECU 15 executes the white smoke reduction control if none of theabove conditions (1) to (7) is satisfied, and executes the white smokesuppression process if any of the above conditions (1) to (7) issatisfied. Alternatively, the ECU 15 may execute the white smokesuppression process (S7) if a specific combination of the aboveconditions (1) to (7) are satisfied. For example, the ECU 15 may executethe white smoke suppression process if the headlight is on and it isnighttime. Further, the ECU 15 may execute the white smoke suppressionprocess if the wiper is on and the detected ambient humidity exceeds thepredetermined value A. Then, the ECU 15 may execute the white smokereduction process (S6) if the specific combination of the aboveconditions (1) to (7) are satisfied.

In the first embodiment, if it is determined in S3 of FIG. 2 that thevehicle speed is not 0, or that the shift lever is not in the Pposition, the process returns to S1. That is, in this case, neither ofthe white smoke reduction control and the white smoke suppressioncontrol is executed. Alternatively, the ECU 15 may first perform thedetermination in S5 of FIG. 2, that is, determine whether thesurrounding environmental conditions are satisfied. Then, in the casewhere none of the above conditions (1) to (7) is satisfied, the ECU 15may return the process to S1. That is, the ECU 15 may execute neither ofthe white smoke reduction control and the white smoke suppressioncontrol. Conversely, if any of the above conditions (1) to (7) issatisfied, the ECU 15 may determine whether the vehicle speed is 0, orwhether the shift lever is in the P position, and execute either thewhite smoke reduction control or the white smoke suppression controlaccording to the determination results.

In the above first embodiment, the necessity for white smoke reductionis determined, and either the white smoke reduction control or the whitesmoke suppression control is executed, depending mainly on whether whitesmoke is highly visible from outside the vehicle. Alternatively, the ECU15 may determine the necessity for white smoke reduction, and executeeither the white smoke reduction control or the white smoke suppressioncontrol, depending on whether the vision of the driver of the vehicle isaffected. FIG. 3 shows an example of such a process.

In this process, the ECU 15 first acquires the gas temperature at theexhaust exit detected by the discharge gas temperature sensor 17 and thetemperature detected by the ambient temperature sensor 19A. Then, theECU 15 determines whether the difference between the gas temperature atthe exhaust exit and the ambient temperature exceeds a predeterminedvalue T1 (S1). If the difference between the gas temperature at theexhaust exit and the ambient temperature does not exceed thepredetermined value T1 (NO in S1), the ECU 15 returns the process to S1.On the other hand, if the difference between the gas temperature at theexhaust exit and the ambient temperature exceeds the predetermined valueT1 (YES in S1), the ECU 15 determines the surrounding environmentalconditions (S4). These conditions are the same as in the firstembodiment, and therefore their descriptions are omitted.

Then, the ECU 15 determines whether the surrounding environmentdetermined in S4 satisfies a predetermined condition (S5). If none ofthe surrounding environmental conditions (the above conditions (1) to(7)) is satisfied (NO in S5), the ECU 15 returns the process to S1. Onthe other hand, if any of the surrounding environmental conditions (theabove conditions (1) to (7)) are satisfied (YES in S5), the ECU 15 readsthe position of the shift lever and a switch signal of the parking brake(S8).

In S9 that follows, the ECU 15 determines whether the shift lever is inthe R position (reverse position), or whether the parking brake signalis off. At this time, the ECU 15 may determine whether the shift leveris in the R position and whether the parking brake signal is off.

Then, if the shift lever is not in the R position, or if the parkingbrake signal is on (NO in S9), the ECU 15 executes the white smokereduction control (S10). In this case, the vehicle does not move in thedirection in which gas is discharged from the exhaust exit, andtherefore the ECU 15 determines that the possibility that the vision ofthe driver is affected is low, even under conditions where white smokeis generated and its visibility is relatively high. In this case, theECU 15 determines that the white smoke reduction process is sufficient.

Alternatively, if the shift lever is in the R position, or if theparking brake signal is off (YES in S9), the ECU 15 determines that thepossibility that the vehicle is moving in the direction in which gas isdischarged from the exhaust exit is high. In this case, the ECU 15determines not only that the white smoke is easily viewable from outsidethe vehicle, but also that the white smoke can affect the vision of thedriver, and thus executes the white smoke suppression control (S11).

As discussed above, in this modification, the ECU 15 estimates theinfluence of white smoke on the driver by determining whether theadvancing direction of the vehicle coincides with the dischargingdirection of gas. Then, the ECU 15 executes the white smoke suppressioncontrol in the case where the advancing direction of the vehiclecoincides with the discharging direction of gas.

In the white smoke suppression control, the ECU 15 stops scavenging air,suspends the operation of the fuel cell system, and/or the like. On theother hand, in the white smoke reduction control, the ECU 15 restrictsscavenging air, heats the cathode off gas with a heater, and/or thelike.

According to such control, the ECU 15 executes the white smoke reductioncontrol when the visibility of the white smoke is high, and executes thewhite smoke suppression process when the white smoke affects the visionof the driver, allowing efficient utilization of the fuel cell system.For example, it is possible to very precisely determine whether to stopscavenging air for the fuel cell stack 1 or to suspend the operation ofthe fuel cell system.

In the above embodiment, the ECU 15 determines in S3 of FIG. 2 whetherthe vehicle speed is 0, or whether the shift lever is in the P position.However, the white smoke reduction process or the white smokesuppression process may be executed when the vehicle speed is at orbelow a predetermined speed, from the standpoint of the merchantabilityor upon user request. In this case, the predetermined speed may be setas factory default, adjusted by the dealer, or set by the user. If thevehicle speed is at or below (or below) the predetermined speed, the ECU15 executes the processes in S4 to S7 of FIG. 2.

Referring to FIGS. 4 and 5, a fuel cell system in accordance with asecond embodiment of the present invention will be described. In theabove first embodiment, the ECU 15 does not execute white smokereduction in the case where the vehicle speed is not 0, or in the casewhere the shift lever is not in the P position. Further, in the casewhere the vehicle speed is 0, or in the case where the shift lever is inthe P position, the ECU 15 selectively executes the white smokereduction control or the white smoke suppression control depending onthe status of whether the surrounding environmental conditions (1) to(7) are satisfied. Alternatively, the fuel cell system in accordancewith the second embodiment of the present invention executes white smokereduction in more diversified levels. In the description below, whitesmoke reduction is executed in white smoke reduction control 1, whitesmoke reduction control 2, and white smoke reduction control 3 (whitesmoke suppression control). The other configuration and function of thisembodiment are the same as those of the first embodiment. Thus, likeprocesses are denoted by like reference numerals to omit theirdescriptions. The system configuration is the same as that shown in FIG.1.

FIG. 4 shows the classification of the white smoke reduction process bythe fuel cell system in accordance with this embodiment. In the whitesmoke reduction control 1, the scavenging air is reduced by 30%, and thepower for heating the cathode off gas is set to 300 watts, for example.Reducing the scavenging air by 30% means reducing by 30% the drivingpower for the pump 9 at the air electrode side. In the white smokereduction control 2, the scavenging air is reduced by 60%, and the powerfor heating the cathode off gas is set to 600 watts, for example. In thewhite smoke reduction control 3, the scavenging air is reduced by 100%,and the power for heating the cathode off gas is set to 1000 watts, forexample. That is, in the white smoke reduction control 3, the pump 9 isstopped to stop the scavenging air. In this case, the operation of thefuel cell system itself may be stopped by shutting off the hydrogenpressure with the pressure regulator 10, for example. In this way, theECU 15 compulsorily suppresses the generation of white smoke.

FIG. 5 shows the control by the ECU 15 in the fuel cell system. Thesteps S1, S2, and S3 in this process are the same as those in the firstembodiment, and therefore their descriptions are omitted.

If the determination in S3 is NO, the ECU 15 executes the white smokereduction control 1 (S6A). On the other hand, if the determination in S3is YES, the ECU 15 determines the surrounding environmental conditions(S4). The surrounding environmental conditions determined in S4 areidentical to the conditions (1) to (7) described in relation to thefirst embodiment.

Then, if none of the conditions (1) to (7) is satisfied, the ECU 15executes the white smoke reduction control 2 (S6B). However, if any ofthe conditions (1) to (7) is satisfied, the ECU 15 executes the whitesmoke reduction control 3 (S6C). That is, the scavenging air for thefuel cell stack 1 is stopped, or the fuel cell system is suspended tosuppress the generation of white smoke.

As discussed above, according to the fuel cell system in accordance withthis embodiment, white smoke reduction can be executed in morediversified levels than in the first embodiment.

As described in relation to the modification of the first embodiment,the ECU 15 may execute the white smoke suppression process (S7) when aspecific combination of the above conditions (1) to (7) are satisfied.For example, the ECU 15 may execute the white smoke suppression processif the headlight is on and it is nighttime. Further, the ECU 15 mayexecute the white smoke suppression process if the wiper is on and thedetected ambient humidity exceeds the predetermined value A.

In addition, as described in relation to FIG. 3, the ECU 15 maydetermine the necessity for white smoke reduction, and execute eitherthe white smoke reduction control or the white smoke suppressioncontrol, depending on whether the vision of the driver of the vehicle isaffected. FIG. 6 shows an example of such a process.

In this process, when the surrounding environment satisfies none of theconditions (1) to (7)) (NO in S5), the ECU 15 executes the white smokereduction control 1 (S10A). On the other hand, if any of the surroundingenvironmental conditions (the above conditions (1) to (7)) are satisfied(YES in S5), the ECU 15 reads the position of the shift lever and aswitch signal of the parking brake (S8). Subsequently, the ECU 15determines whether the shift lever is in the R position, or whether theparking brake signal is off.

Then, if the shift lever is not in the R position, or if the parkingbrake signal is on (NO in S9), the ECU 15 executes the white smokereduction control 2 (S10B). On the other hand, if the shift lever is inthe R position, or if the parking brake signal is off (YES in S9), theECU 15 determines that the possibility that the vehicle moves in thedirection in which gas is discharged from the exhaust exit is high. Inthis case, the ECU 15 determines not only that the white smoke is easilyviewable from outside the vehicle, but also that the white smoke canaffect the vision of the driver, and executes the white smokesuppression control (S11).

Referring to FIGS. 7 to 9, a third embodiment of the present inventionwill be described. In the first and second embodiments, the ECU 15determines whether white smoke reduction is necessary based on thedifference between the gas temperature at the exhaust exit and theambient temperature (see the process in S1 of FIGS. 2, 3, 5, and 6).Alternatively, the ECU 15 may determine whether white smoke reduction isnecessary based on the surrounding environmental conditions of thevehicle, which include the ambient temperature and the ambient humidity,and the gas temperature at the exhaust exit. The other configuration andfunction of this embodiment are the same as those of the first andsecond embodiments. Like components are denoted by like referencenumerals to omit their descriptions.

FIG. 7 is a flowchart illustrating the operation of the fuel cell systemin accordance with the embodiment of the present invention.

The ECU 15 first acquires the temperature of the cathode off gasdetected by the discharge gas temperature sensor 17 (S1A). The ECU 15also acquires the ambient temperature detected by the ambienttemperature sensor 19A (S1A). The ECU 15 further acquires the ambienthumidity detected by the ambient humidity sensor 19B (S1A). In thisembodiment, the ambient humidity measured by the ambient humidity sensor19B is referred to as “detected ambient humidity.”

Next, the ECU 15 makes a determination as to a white smoke reductionregion based on the temperature of the cathode off gas, the detectedambient temperature, and the detected ambient humidity (S1B).

In S1B, the term “white smoke reduction region” refers to the statewhere water vapor contained in the cathode off gas discharged to theambient air is white and visible. The relationship between the whitesmoke reduction region and the temperature of the cathode off gas, theambient temperature, and the ambient humidity may be obtained in advanceby experiment or by simulation. For example, a map (table) as shown inFIG. 8 is prepared in advance by experiment or by simulation. Then, theECU 15 may make a determination as to the white smoke reduction regionusing the map.

The symbol ΔT indicated in FIG. 8 represents the difference between thetemperature of the cathode off gas and the ambient temperature. The term“humidity” indicated in FIG. 8 refers to the ambient humidity. Thecircular marks in FIG. 8 indicate that ΔT1 and the ambient humidity arein the white smoke reduction region. The X marks in FIG. 8 indicate thatΔT1 and the ambient humidity are not in the white smoke reductionregion. If the map in FIG. 8 is used to determine whether ΔT1 and theambient humidity are in the white smoke reduction region, the ECU 15calculates ΔT1, the difference between the temperature of the cathodeoff gas and the ambient temperature. Then, the ECU 15 references the mapshown in FIG. 8 to determine whether ΔT1 and the ambient humidity are inthe white smoke reduction region (S1B).

If ΔT1 and the ambient humidity are determined to be in the white smokereduction region (YES in S1B), the ECU 15 executes the processes in andafter S2. The processes in and after S2 are the same as those in thefirst embodiment and the second embodiment, and therefore theirdescriptions are omitted. On the other hand, if the ECU 15 determinesthat ΔT1 and the ambient humidity are not in the white smoke reductionregion (NO in S1B), the process is returned to S1A.

According to this embodiment, it is possible to determine at an initialstage whether white smoke reduction is required in consideration of theambient humidity as well:

In addition, in and after S2 of FIG. 7, the same processes as those inand after S2 of FIG. 2 are executed. Alternatively, in and after S2 ofFIG. 7, the same processes as those in and after S4 of FIG. 3 may beexecuted. As a further alternative, the same processes as those in andafter S2 of FIG. 5, or those in and after S4 of FIG. 6, of the secondembodiment may be executed in and after S2 of FIG. 7.

Referring to FIGS. 9 to 11, a fuel cell system in accordance with afourth embodiment of the present invention will be described. In thefirst to third embodiments, when white smoke reduction is determined tobe necessary, then the ECU 15 executes the white smoke reduction controlor the white smoke suppression control depending on whether white smokeis highly visible from outside the vehicle, or whether white smokeaffects the vision of the driver.

In this embodiment, the fuel cell system dilutes the cathode off gascontaining water vapor and having passed through the fuel cell stack 1with air that has not passed through the fuel cell stack 1. In thiscase, the dilution ratio with air containing water vapor as the targetof control is determined using an evaluation expression reflecting thespeed of the vehicle. The other configuration and function are the sameas those of the first to third embodiments. Like components are denotedby like reference numerals to omit their descriptions.

FIG. 9 is a configuration diagram of a fuel cell system in accordancewith this embodiment. In FIG. 9, compared to the configuration of FIG.1, an air back pressure regulator 31 and a diluter 32 are added at thecathode side. The cathode off gas passage 6 is connected to the diluter32 from the cathode side.

At the anode side, an anode off gas branch passage 37 is connected tothe anode off gas passage 3 via an exhaust valve 36. The anode off gasbranch passage 37 is also connected to the diluter 32. Furthermore, inFIG. 9, a shut valve 30 is provided upstream of the pressure regulator10 for hydrogen.

The ECU 15 controls the air backpressure regulator 31, the exhaust valve36, and the pump 9 to control the dilution ratio of the anode off gasand the cathode off gas.

FIG. 10 shows the concept of a vehicle equipped with the fuel cellsystem. In FIG. 10, the vehicle advances in the direction of the arrowat a speed v. The flow rate of the cathode off gas discharged from thecathode off gas passage 6 to the diluter 32 at this time is defined asQairex.

In this case, the dilution ratio Dr of the cathode off gas at thevehicle speed v, which reflects the effect that the cathode off gas isdiffused by the ambient air, is defined by Expression 1 below.

$\begin{matrix}{{{Dilution}\mspace{14mu} {ratio}\text{:}\mspace{14mu} {Dr}} = \frac{\frac{P_{H\; 2{O@{FC}}}}{P_{H\; 2{O@{atm}}}} \times Q_{{air}_{ex}}}{Q_{{air}_{ex}} + {k \cdot v}}} & \left\lbrack {{Expression}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Where,

k: Conversion factor to obtain dilution air amountQair_(ex): Amount of exhaust airP_(H2O@FC): Saturated water vapor pressure at operating temperature ofFCP_(H2O@atm): Saturated water vapor pressure at ambient temperature

In the expression, k represents a conversion factor for calculating theamount of air with which the cathode off gas is diluted at the vehiclespeed v, P_(H2O@FC) is a saturated vapor pressure at the operatingtemperature of the fuel cell stack 1, and P_(H2O@atm) is a saturatedvapor pressure at the ambient temperature.

In the physical sense, Expression 1 means that the concentration of thecathode off gas is lowered by dilution as the vehicle speed v is higher.That is, if the saturated vapor pressure at the operating temperature ofthe fuel cell stack 1 is higher than the saturated vapor pressure at theambient temperature, P_(H2O@FC)/P_(H2O@atm) is more than 1, whichsatisfies the condition under which white smoke is generated. Here,P_(H2O@FC)/P_(H2O@atm) represents the proportion of the cathode off gasthat is condensed into water droplets when the gas is released to theambient air. In addition, (P_(H2O@FC)/P_(H2O@atm))×Qairex represents theamount of the cathode off gas that is condensed into water droplets whenthe gas is released to the ambient air.

When the vehicle speed v is sufficiently high, however, white smoke isnot noticeable due to the same effect as when the cathode off gas isdiluted. The dilution ratio Dr indicates the degree of such effect.

Thus, P_(H2O@FC)/P_(H2O@atm) is set to a plurality of values, and foreach value, the degree of the generation of white smoke is observedwhile varying the cathode off gas flow rate Qairex and the vehicle speedv. Based on the values obtained from such experiments, the relationshipbetween P_(H2O@FC), P_(H2O@atm), Qairex, and the vehicle speed v and thedegree of the generation (visibility) of white smoke may be obtained.From the values obtained from the experiments, the value to be fulfilledby the dilution ratio Dr (hereinafter referred to as “reference valueDr0”) is determined. Then, with the reference value Dr0 set in a memory(not shown) of the ECU 15, the operating state of the fuel cell may becontrolled such that the dilution ratio Dr is at the reference value Dr0or less.

FIG. 11 is a flowchart showing the control executed by the ECU 15 inthat case. In this process, the ECU 15 first reads the operatingtemperature of the fuel cell stack 1 from the stack temperature sensor20 to calculate the saturated vapor pressure P_(H2O@FC) of the cathodeoff gas (S21). However, the exhaust gas temperature sensor 17 of FIG. 1may be used in place of the stack temperature sensor 20 to detect thetemperature of the cathode off gas.

Next, the ECU 15 reads the ambient temperature from the ambienttemperature sensor 19A to calculate the saturated vapor pressure of theambient air P_(H2O@atm) (S22). Further, the ECU 15 reads the vehiclespeed v from a vehicle speed sensor (not shown) (S23). Then, the ECU 15determines the cathode off gas flow rate Qairex such that the dilutionratio Dr is at the reference value Dr0 or less according to Expression 1(S24). Then, the ECU 15 controls the rotational speed of the pump 9 suchthat the cathode off gas flow rate is at Qairex or less (S25).

As discussed above, according to the fuel cell system in accordance withthe fourth embodiment, the ECU 15 controls the discharge amount of thecathode off gas, that is, the rotational speed of the pump 9, such thatthe dilution ratio is at the reference value Dr0 or less based on thesaturated vapor pressure of the cathode off gas, the saturated vaporpressure of the ambient air, and the vehicle speed. According to suchcontrol, white smoke due to the cathode off gas may be controlled so asnot to be noticeable by controlling the discharge amount, that is, thedilution ratio, of the cathode off gas according to the vehicle speed v.

In the fourth embodiment, the ECU 15 controls the discharge amount, thatis, the dilution ratio, of the cathode off gas according to the vehiclespeed v to suppress white smoke. In such control, the ECU 15 may furthercontrol the backpressure at the cathode side. The backpressure at thecathode side is controlled by the opening of the backpressure regulator31 at the exit of the flow path of the fuel cell stack 1 at the cathodeside.

In the fuel cell system, the ECU 15 calculates the amount in mass of thecathode off gas that is discharged. For example, it is assumed thatpower is generated while the cathode off gas is required that isdischarged in an amount of M gram. As the backpressure at the cathodeside is higher, the volume of the off gas that is discharged is smallerfor the same mass. For example, when the back pressure doubles, therequired flow rate of the off gas that is discharged is halved.

On the other hand, the saturated vapor pressure relies upon thetemperature, rather than the backpressure, of the off gas, and thereforea generally constant amount of water vapor is contained in the off gasof the same volume, regardless of the backpressure. Thus, when the backpressure doubles, the required volumetric flow rate of the off gas thatis discharged is halved. Thus, at this time, the amount of water vaporto be discharged together with the off gas is also halved.

With the influence of the backpressure reflected in the dilution ratiogiven by Expression 1, Expression 2 below can be obtained.

$\begin{matrix}{{{{Dr} = \frac{\frac{P_{H\; 2{O@{FC}}}}{P_{H\; 2{O@{atm}}}} \times \left( {Q_{{air}_{ex}} \times \frac{P_{atm}}{P_{back}}} \right)}{Q_{{air}_{ex}} + {k \cdot v}}}{Q_{{air}_{ex}} \times \frac{P_{atm}}{P_{back}}\text{:}}}\mspace{14mu}} & \left\lbrack {{Expression}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Changes in volumetric flow rate brought about by increasing backpressure

In the expression, Patm represents the atmospheric pressure, Pbackrepresents the backpressure at the cathode side, and Qair×(Patm/Pback)represents changes in volumetric flow rate brought about by increasingthe backpressure. Thus, for the same amount (in mass) of the off gas tobe discharged, the dilution ratio may be reduced by the changes involumetric flow rate. Conversely, during operation when the backpressureis reduced, the generation of white smoke may be reduced by reducing theamount in mass of the off gas to be discharged.

As described in the above modification 1, the ECU 15 may reduce thegeneration of white smoke by further controlling the back pressure inaddition to performing the process of FIG. 11. If the control of thebackpressure is given priority, the ECU 15 may increase and reduce theamount in mass of the off gas to be discharged according to changes inback pressure.

In the fourth embodiment, the ECU 15 controls the discharge amount, thatis, the dilution ratio, of the cathode off gas according to the vehiclespeed v to suppress the generation of white smoke. In this case, theinfluence of winds around the vehicle may be reflected in the vehiclespeed v. For example, wind pressure sensor may be disposed at fourpositions around the vehicle, namely, at the front, rear, left, andright sides of the vehicle, to calculate the relative speed V betweenthe vehicle and the ambient air based on the wind pressures,irrespective of whether the vehicle is stationary or traveling. As therelative speed V between the vehicle and the ambient air, the larger oneof the relative speed V1 in the longitudinal direction, which is basedon the wind pressure in the longitudinal direction of the vehicle, andthe relative speed V2 in the lateral direction, which is based on andthe lateral direction of the vehicle, may be used.

Then, the ECU 15 may calculate the dilution ratio of Expression 1 basedon the relative speed V between the vehicle and the ambient air in placeof the vehicle speed v. In this way, while the fuel cell systemsuppresses the generation of white smoke, the limit amount of thecathode off gas can be discharged is increased, even when the vehicle istraveling at a low speed or stationary, by calculating the dilutionratio with the relative speed between the vehicle and the ambient airtaken into account.

While the invention has been described with reference to exampleembodiments thereof, it is to be understood that the invention is notlimited to the described embodiments or constructions. To the contrary,the invention is intended to cover various modifications and equivalentarrangements. In addition, while the various elements of the exampleembodiments are shown in various combinations and configurations, othercombinations and configurations, including more, less or only a singleelement, are also within the spirit and scope of the invention.

1. An exhaust state control device for a fuel cell for a mobile unitthat detects a state of exhaust gas from a fuel cell mounted on a mobileunit, comprising: an exhaust gas temperature sensor that measures, as astate of the exhaust gas, a temperature of the exhaust gas in adischarge passage that discharges the exhaust gas from a main body ofthe fuel cell; an ambient temperature sensor that measures, as a stateof the ambient air, a temperature of the ambient air to which theexhaust gas is to be discharged; a motion sensor that detects movementof the mobile unit; a white smoke generation determination section thatdetermines whether white smoke is generated based on a relationshipbetween the state of exhaust gas and the state of ambient air; acondition determination section that determines whether a predeterminedcondition to reduce white smoke is satisfied based on a detection signalfrom the motion sensor when the white smoke generation determinationsection determines that white smoke is generated; and a control sectionthat activates a process to reduce white smoke if the conditiondetermination section determines that the predetermined condition issatisfied.
 2. The exhaust state control device for a fuel cell accordingto claim 1, further comprising: an operation state sensor that detectsan operating state of an operating section that operates the mobileunit; and a sensor that senses a surrounding environmental state of themobile unit, wherein the condition determination section determineswhether the predetermined condition to reduce white smoke is satisfiedbased on at least one of the detected moving state of the mobile unit,the detected operating state of the operating section that operates themobile unit, and the detected surrounding environmental state of themobile unit.
 3. The exhaust state control device for a fuel cellaccording to claim 1, wherein the control section activates a process toreduce white smoke when the condition determination section determinesthat a first predetermined condition is satisfied, and activates aprocess to further reduce generation of white smoke than when the firstpredetermined condition is determined to be satisfied, if the conditiondetermination section determines that a second predetermined conditionis satisfied.
 4. The exhaust state control device for a fuel cellaccording to claim 2, wherein the control section activates a process toreduce white smoke when the condition determination section determinesthat a first predetermined condition is satisfied, and activates aprocess to further reduce generation of white smoke than when the firstpredetermined condition is determined to be satisfied, if the conditiondetermination section determines that a second predetermined conditionis satisfied.
 5. The exhaust state control device for a fuel cellaccording to claim 4, wherein the white smoke generation determinationsection determines whether white smoke is generated according to thedifference between the exhaust gas temperature and the ambient airtemperature.
 6. The exhaust state control device for a fuel cellaccording to claim 4, further comprising an ambient humidity sensor thatmeasures a humidity of the ambient air to which the exhaust gas is to bedischarged, wherein the white smoke generation determination sectiondetermines whether white smoke is generated according to the differencebetween the exhaust gas temperature and the ambient air temperature, andthe ambient air humidity.
 7. The exhaust state control device for a fuelcell according to claim 4, wherein: the first predetermined condition issatisfied if either one of the conditions, where a moving speed of themobile unit is determined to be at or below a predetermined value andwhere a visibility of white smoke around the mobile unit is determinedto be higher than that in a reference environment, is satisfied; and thesecond predetermined condition is satisfied if both the conditions,where the moving speed of the mobile unit is determined to be at orbelow the predetermined value and where the visibility of white smokearound the mobile unit is determined to be higher than that in thereference environment, are satisfied.
 8. The exhaust state controldevice for a fuel cell according to claim 7, wherein the condition wherethe moving speed of the mobile unit is determined to be at or below thepredetermined value includes a state where the mobile unit isstationary.
 9. The exhaust state control device for a fuel cellaccording to claim 7, wherein the moving speed of the mobile unit is anabsolute speed of the mobile unit or a relative speed between the mobileunit and the ambient air around the mobile unit.
 10. The exhaust statecontrol device for a fuel cell according to claim 4, wherein: the mobileunit is a vehicle; the first predetermined condition is satisfied ifeither one of conditions, where a shift lever is determined to be in aparking position and where a visibility of white smoke around the mobileunit is determined to be higher than that in a reference environment, issatisfied; and the second predetermined condition is satisfied if boththe conditions, where the shift lever is determined to be in the parkingposition and where the visibility of white smoke around the mobile unitcan be determined to be higher than that in the reference environment,are satisfied.
 11. The exhaust state control device for a fuel cellaccording to claim 4, wherein: the first predetermined condition issatisfied if either one of conditions, a condition where the white smokegeneration determination section determines that white smoke isgenerated, or a condition where a moving speed of the mobile unit is ator below a predetermined value and where a visibility of white smokearound the mobile unit is determined to be lower than that in areference environment, is satisfied; and the second predeterminedcondition is satisfied if both the conditions, where the moving speed ofthe mobile unit is determined to be at or below the predetermined valueand where the visibility of white smoke around the mobile unit isdetermined to be higher than that in the reference environment, aresatisfied.
 12. The exhaust state control device for a fuel cellaccording to claim 11, wherein if the conditions, where the white smokegeneration determination section determines that white smoke isgenerated, and where the moving speed of the mobile unit is at or belowthe predetermined value and the visibility of white smoke around themobile unit is determined to be lower than that in the referenceenvironment, are satisfied, the process to reduce white smoke, which isperformed when the condition determination section determined that thefirst predetermined condition has been satisfied, is performed indifferent level.
 13. The exhaust state control device for a fuel cellaccording to claim 4, wherein: the first predetermined condition issatisfied if either one of conditions, where a vision of an operator whooperates the mobile unit is determined to be affected by white smoke andwhere a visibility of white smoke around the mobile unit is higher thanthat in a reference environment, is satisfied; and the secondpredetermined condition is satisfied if both the condition, where thevision of the operator who operates the mobile unit is determined to beaffected by white smoke and where the visibility of white smoke aroundthe mobile unit is higher than that in the reference environment, aresatisfied.
 14. The exhaust state control device for a fuel cellaccording to claim 7, wherein the reference environment includes atleast one of a daytime environment and a non-rainy environment.
 15. Theexhaust state control device for a fuel cell according to claim 14,wherein: the mobile unit is a vehicle; the operating state of theoperating section is determined based on whether a headlight is on; thesurrounding environmental state is determined based on at least one of:whether a darkness detection signal for an automatic light is on;whether a current time is after a sunset and before a sunrise; whether asignal from a global positioning system cannot be received; whether awiper is on; whether a rain sensing signal is on; and whether adetection value of the ambient humidity sensor exceeds a predeterminedvalue; and the condition determination section determines that thereference environment is the daytime or non-rainy environment if atleast one of the determined states above is satisfied.
 16. The exhauststate control device for a fuel cell according to claim 13, wherein thevision of the operator is determined to be affected by white smoke ifthe mobile unit advances in a discharging direction of the exhaust gas.17. The exhaust state control device for a fuel cell according to claim13, wherein: the mobile unit is a vehicle; and it is determined that thecondition where the white smoke interferes with vision of the operatorof the mobile unit is satisfied when at least one of the shift lever isin a reverse position and the parking brake is off.
 18. The exhauststate control device for a fuel cell according to claim 4, wherein theprocess to reduce white smoke is at least one of a process to controlthe exhaust gas temperature and a process to control a flow rate of theexhaust gas at an air electrode side of the fuel cell.
 19. The exhauststate control device for a fuel cell according to claim 18, whereindischarge of the exhaust gas at the air electrode side is controlledsuch that a limit of the flow rate of the exhaust gas decreases as themoving speed of the mobile unit decreases.
 20. The exhaust state controldevice for a fuel cell according to claim 18, wherein a pressure or theflow rate of the exhaust gas at the air electrode side is controlledsuch that the limit of the flow rate of the exhaust gas increases as thepressure of the exhaust gas increases.
 21. The exhaust state controldevice for a fuel cell according to claim 4, wherein the process toreduce the generation of white smoke, which is performed when thecondition determination section determines that the second predeterminedcondition is satisfied, includes at least one of stopping scavenging airand shutting off a hydrogen supply pressure to the fuel cell.